Mass Extinction 252M Years Ago Made Space For Early Dinosaur Forerunners

April 30, 2013

Image Caption: The pig-size Dicynodon was part of a large, dominant group of plant eaters found across the southern hemisphere until the mass extinction event weakened their numbers so that newly emerging herbivores could compete. | Image Credit: Marlene Donnelly/Field Museum of Natural History

Approximately 252 million years ago, during the world´s largest mass extinction event, nine out of ten species vanished from the planet. Based on fossil records from sites in South Africa and southwest Russia, many scientists have long thought the predecessors of dinosaurs largely missed the race to fill habitat niches that were emptied during this event.

However, according to an international team of scientists, it turns out they may have been looking for the starting line in the wrong places. The findings of this study were published in a recent issue of Proceedings of the National Academy of Sciences.

The new findings are based on recently discovered fossils from 10 million years that followed the mass-extinction event. They reveal a lineage of animals thought to have led to dinosaurs taking hold in Tanzania and Zambia in the mid-Triassic period, many millions of years before dinosaur relatives were seen in the fossil record elsewhere on Earth.

“The fossil record from the Karoo of South Africa remains a good representation of four-legged land animals across southern Pangea before the extinction event. But after the event, animals weren’t as uniformly and widely distributed as before. We had to go looking in some fairly unorthodox places,” said Christian Sidor, a biology professor at the University of Washington (UW).

Seven new fossil-hunting expeditions conducted since 2003 in Tanzania, Zambia and Antarctica were combined with work combing through and reexamining fossil collections to provide the new insights. The expeditions were funded by the National Geographic Society and the National Science Foundation (NSF). The team created two “snapshots” of four legged-animals approximately 5 million and again at 10 million years before the extinction event at the end of the Permian period.

For example, “¯the pig-sized Dicynodon“¯— said to resemble a fat lizard with a short tail and turtle’s head — was a dominant plant-eating species distributed across southern Pangea prior to the extinction event. Pangea is the name scientists have given to the enormous landmass that existed when all the world´s continents were joined together. Southern Pangea consisted of what is currently Africa, South America, Antarctica, Australia and India. After the extinction event, Dicynodon“¯disappeared completely, and other related species were so greatly diminished in numbers that newly emerging herbivores were suddenly able to compete with them.

“Groups that did well before the extinction didn’t necessarily do well afterward,” Sidor said. “What we call evolutionary incumbency was fundamentally reset.”

Among other things, the snapshot 10 million years after the event reveals archosaurs — which include crocodiles, birds and a variety of now extinct animal forms — were in Tanzanian and Zambian basins but were not distributed across the broader area of southern Pangea as was the previous pattern for four-legged animals. Archosaurs are of particular interest to researchers because they think they may have led to dino-like animals like Asilisaurusand and“¯Nyasasaurusparringtoni. The latter of these was a dog-sized creature with a five-foot-long tail that scientists announced in December 2012 could be the earliest dinosaur, or else the closest relative found so far.

“Early archosaurs being found mainly in Tanzania is an example of how fragmented communities became after the extinction event,” Sidor said. And the co-authors write: “These findings suggest that “¦ archosaur diversification was more intimately related to recovery from the end-Permian mass extinction than previously suspected.”

UW biology graduate student Daril Vilhena recently developed a new framework for analyzing biogeographic patterns from species distribution. This new method provides a way to discern the complex recovery after the extinction event.

Vilhena´s framework revealed that before the event, 35 percent of four-legged species were found in two or more of the five areas studied. Some of the species had ranges that stretched 1,600 miles, encompassing the Tanzanian and South African basins. The authors say ten million years after the event, clear geographical clustering occurred, and just 7 percent of the species were found in two or more regions.

The new framework — which includes new ways to statistically consider how connected or isolated species are from one another — could be useful for modern day biogeographers as well as paleontologists.